Manufacturing method of light emitting device

ABSTRACT

A manufacturing method of a light emitting device is provided. A first electrode is formed on a substrate. The first electrode includes a patterned conductive layer, and the patterned conductive layer includes an alloy containing a first metal and a second metal. An annealing process is performed on the first electrode, so as to form a passivation layer at least on a side surface of the first electrode. The passivation layer includes a compound of the second metal. A light emitting layer is formed on the first electrode. A second electrode is formed on the light emitting layer.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional application of and claims the prioritybenefit of U.S. application Ser. No. 13/402,882, filed on Feb. 23, 2012,now pending, which claims the priority benefit of Taiwan applicationserial no. 100146506, filed on Dec. 15, 2011. The entirety of each ofthe above-mentioned patent applications is hereby incorporated byreference herein and made a part of specification.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a manufacturing method of a device. Moreparticularly, relates to a manufacturing method of a light emittingdevice.

2. Description of Related Art

An organic electro-luminescence device is characterized byself-luminance, high brightness, high contrast, wide view angle, fastresponse speed, and so on. Therefore, among various displays, an organicelectro-luminescence display (OELD) panel frequently draws attention inthe industry. Generally, the OELD panel can be classified into a topemission OELD panel and a bottom emission OELD panel.

Silver or any other metal with high reflectivity is often applied toform an anode in the top emission OELD panel, so as to enhanceelectro-luminescence intensity. However, a work function of silver isoverly low, and thus metal oxide (e.g., indium tin oxide, ITO) with thehigh work function is often required to form an anode with a stackedstructure containing ITO (bottom)/silver (middle)/ITO (top), so as tomatch the anode with a work function of a hole injection layer in theorganic electro-luminescence device. That is to say, in the process offorming the anode, the ITO layer, the silver layer, and the ITO layerneed be respectively etched to pattern the anode. It should be mentionedthat a sidewall of the silver layer is exposed after the anode iscompletely etched, and the exposed sidewall of the silver layer is aptto react with a stripper in the subsequent process. Thereby, the exposedsidewall of the silver layer is corroded. Specifically, sulfide in thestripper reacts with silver, and the resultant sulfidization not onlyleads to the formation of black silver sulfide (Ag₂S) around the anodebut also continues to damage the anode.

SUMMARY OF THE INVENTION

The invention is directed to a manufacturing method of a light emittingdevice for forming a passivation layer on a sidewall of an electrode, soas to prevent exposure of the electrode and allow the light emittingdevice to have favorable device properties.

In this invention, a manufacturing method of a light emitting device isprovided. In the manufacturing method, a first electrode is formed on asubstrate. The first electrode includes a first patterned conductivelayer, and the first patterned conductive layer includes an alloycontaining a first metal and a second metal. An annealing process isperformed on the first electrode, so as to form a passivation layer atleast on a side surface of the first electrode. The passivation layerincludes a compound of the second metal. A light emitting layer isformed on the first electrode. A second electrode is formed on the lightemitting layer.

Based on the above, in the light emitting device described in anembodiment of the invention, the electrode has a proper work function,thus allowing the light emitting device to have favorable deviceproperties and light emitting intensity. Moreover, in the light emittingdevice and the manufacturing method thereof, as described in anotherembodiment of the invention, the passivation layer is formed on the sidesurface of the electrode, so as to prevent exposure of the electrode.Thereby, the electrode is not damaged, and the light emitting device canthus have favorable device properties.

In order to make the aforementioned and other features and advantages ofthe invention more comprehensible, embodiments accompanying figures aredescribed in detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide further understanding,and are incorporated in and constitute a part of this specification. Thedrawings illustrate exemplary embodiments and, together with thedescription, serve to explain the principles of the invention.

FIG. 1 is a schematic cross-sectional view illustrating a light emittingdevice according to an embodiment of the invention.

FIG. 2A to FIG. 2E are schematic cross-sectional flow chartsillustrating a manufacturing method of a light emitting device accordingto an embodiment of the invention.

FIG. 3A to FIG. 3D are schematic cross-sectional flow chartsillustrating a manufacturing method of a light emitting device accordingto an embodiment of the invention.

DESCRIPTION OF EMBODIMENTS

FIG. 1 is a schematic cross-sectional view illustrating a light emittingdevice according to an embodiment of the invention. With reference toFIG. 1, the light emitting device 100 described in the presentembodiment includes a substrate 102, a first electrode 110, a lightemitting layer 120, and a second electrode 130. In the presentembodiment, the substrate 102 may be a rigid substrate or a flexiblesubstrate. The rigid substrate is, for instance, a glass substrate, arigid plastic substrate, a metal substrate, a wafer, or a ceramicsubstrate; the flexible substrate is, for instance, an organicsubstrate, such as a polyimide (PI) substrate, a polycarbonate (PC)substrate, a polyethylene terephthalate (PET) substrate, a poly(ethylene2,6-napthalate) (PEN) substrate, a polypropylene (PP) substrate, apolyethylene (PE) substrate, a polystyrene (PS) substrate, any otherappropriate substrate, a substrate formed with the above polymerderivates, or a thin metal, thin glass, or thin alloy substrate.

The first electrode 110 is disposed on the substrate 102 and includes analloy containing a first metal and indium, an alloy containing the firstmetal and zinc, or an alloy containing the first metal and indium-zinc.Indium, zinc, or indium-zinc accounts for about 0.1 wt %˜about 2 wt % ofthe alloy. According to the present embodiment, indium, zinc, orindium-zinc accounts for about 0.2 wt %˜about 1 wt % of the alloy, forinstance. The first metal described herein has high reflectivity, suchas silver, aluminum, or the like. The alloy containing the first metaland indium, zinc, or indium-zinc is, for instance, silver-indium alloy,aluminum-indium alloy, silver-zinc alloy, aluminum-zinc alloy,silver-indium-zinc alloy, aluminum-indium-zinc alloy, or any other alloycontaining indium, zinc, or indium-zinc. Based on actual requirements,the alloy containing the first metal and indium, zinc, or indium-zincmay also include other metal. That is to say, the alloy containing thefirst metal and indium, zinc, or indium-zinc may substantially includetwo or more metal elements. Besides, the first electrode 110 in thepresent embodiment exemplarily has a single-layer structure, while thefirst electrode 110 in another embodiment may have a multi-layerstructure, wherein at least one layer of the multi-layer structureincludes the alloy containing the first metal and indium, zinc, orindium-zinc.

The second electrode 130 is disposed on the first electrode 110. Thelight emitting layer 120 is disposed between the first electrode 110 andthe second electrode 130. Here, the light emitting layer 120 may includea red organic light emitting pattern, a green organic light emittingpattern, a blue organic light emitting pattern, a light emitting patternwith other colors, or a combination of the aforesaid light emittingpatterns. The second electrode 130 may be made of a transparentconductive material or a non-transparent conductive material, forinstance. Besides, the second electrode 130 may have a single-layerstructure or a multi-layer structure. The transparent conductivematerial may include metal oxide, such as indium-tin oxide (ITO),indium-zinc oxide (IZO), aluminum-tin oxide (ATO), aluminum-zinc oxide(AZO), indium-germanium-zinc oxide, other suitable oxide (e.g., zincoxide), a stacked layer having at least two of the above materials, anda transparent thin metal, e.g., silver or aluminum with the thicknessless than about 20 nm. The non-transparent conductive material includesmetal, such as silver, aluminum, molybdenum, copper, titanium, or anyother appropriate metal. In the present embodiment, the first electrode110 is the anode, for instance, and the second electrode 130 is thecathode, for instance. However, it should be mentioned that thecondition of the first and second electrodes 110 and 120 beingrespectively the anode and the cathode can be modified based on designrequirements. According to an embodiment (not shown), the light emittingdevice 100 may further include at least one of the hole transportinglayer, the hole injection layer, the electron transporting layer, andthe electron injection layer, so as to improve the light emittingefficiency of the light emitting device. Here, the hole transportinglayer and the hole injection layer are located between the firstelectrode 110 and the light emitting layer 120, for instance, and theelectron transporting layer and the electron injection layer are locatedbetween the second electrode 130 and the light emitting layer 120, forinstance.

In the light emitting device described in the present embodiment, theelectrode includes the alloy containing the first metal and indium,zinc, or indium-zinc, and indium, zinc, or indium-zinc accounts forabout 0.1 wt %˜about 2 wt % of the alloy. Thus, the electrode suitablyserves as the anode of a top emission light emitting device, whichshould however not be construed as a limitation to the invention.Thereby, the device properties and the light emitting intensity of thelight emitting device can be significantly improved. Certainly, based onthe required properties of the electrode, indium, zinc, or indium-zincmay also be applied to form another alloy with another metal, so as toform the electrode and further improve the device properties of thelight emitting device.

FIG. 2A to FIG. 2E are schematic cross-sectional flow chartsillustrating a manufacturing method of a light emitting device accordingto an embodiment of the invention. With reference to FIG. 2A, a firstelectrode material layer 210 is formed on a substrate 202. The firstelectrode material layer 210 includes a first conductive layer 212, andthe first conductive layer 212 includes an alloy containing a firstmetal and a second metal. The first metal described herein has highreflectivity, such as silver, aluminum, or the like. The second metal isindium or zinc, for instance. The alloy containing the first metal andthe second metal is silver-indium alloy, aluminum-indium alloy,silver-zinc alloy, aluminum-zinc alloy, or any other alloy, forinstance. Namely, the first conductive layer 212 includes silver-indiumalloy, aluminum-indium alloy, silver-zinc alloy, aluminum-zinc alloy, orany other alloy, for instance, and the first conductive layer 212 isformed by performing an evaporation process or a sputtering process, forinstance. According to this embodiment, the first electrode materiallayer 210 may further include a second conductive layer 214 locatedbetween the substrate 202 and the first conductive layer 212. The secondconductive layer 214 described herein is made of indium-tin oxide (ITO),zinc-tin oxide (ZTO), indium-zinc oxide (IZO), other suitable materialsand is formed by evaporation, for instance. Based on actualrequirements, the alloy containing the first metal and the second metalmay include other metal. That is to say, the alloy containing the firstmetal and the second metal may substantially include two or more metalelements. For instance, in an embodiment of the invention, the alloycontaining the first metal and the second metal may besilver-indium-zinc alloy or aluminum-indium-zinc alloy. In anotherembodiment, it is likely for the first electrode material layer 210 notto include the second conductive layer 214.

With reference to FIG. 2B, the first electrode material layer 210 ispatterned to form a first electrode 210 a. According to this embodiment,a method of patterning the first electrode material layer 210 includessequentially patterning the first conductive layer 212 and the secondconductive layer 214, for instance. A method of patterning the firstconductive layer 212 is, for instance, performing a photolithography andetching process on the first conductive layer 212 according to thepresent embodiment. If the first conductive layer 212 is exemplarilymade of silver-indium alloy, an etchant for etching the first conductivelayer 212 may be a mixture of phosphoric acid, nitric acid, and aceticacid, for instance. Besides, a method of patterning the secondconductive layer 214 is, for instance, performing a photolithography andetching process on the second conductive layer 214. If the secondconductive layer 214 is exemplarily made of ITO, an etchant for etchingthe second conductive layer 214 may be oxalic acid, for instance.

The first electrode 210 a includes a first patterned conductive layer212 a, and the first patterned conductive layer 212 a includes an alloycontaining the first metal and the second metal. According to thisembodiment, the first electrode 210 a further includes a secondpatterned conductive layer 214 a, for instance, and the second patternedconductive layer 214 a is located between the substrate 202 and thefirst patterned conductive layer 212 a. According to the presentembodiment, the first electrode 210 a has a two-layer structurecontaining ITO/silver-indium alloy, for instance. Notably, the firstelectrode 210 a in the present embodiment exemplarily includes the firstpatterned conductive layer 212 a and the second patterned conductivelayer 214 a, while the second patterned conductive layer 214 a may beomitted in another embodiment of the invention. Hence, steps of formingand patterning the second conductive layer 214 can also be omitted.

With reference to FIG. 2C, an annealing process AP is performed on thefirst electrode 210 a, so as to form a passivation layer 220 at least ona side surface 213 of the first electrode 210 a. Here, the passivationlayer 220 includes a compound of the second metal. Specifically, thesecond metal is separated from the first patterned conductive layer 212a after the annealing process AP is performed, and the second metal mayundergo oxidation or other reactions, so as to form a compound of thesecond metal at least on the side surface 213 of the first electrode 210a. In the present embodiment, the second metal is indium or zinc, forinstance, and the compound of the second metal formed after performingthe annealing process AP is indium oxide (InO_(x)), zinc oxide, orindium-zinc oxide (if the second metal is one of indium and zinc, theother metal is the other one of indium and zinc), for instance. The workfunction of the compound of the second metal ranges from about 4.8 toabout 5.5, for instance. In the present embodiment, the passivationlayer 220 includes InO_(x), zinc oxide, or indium-zinc oxide, forinstance. Besides, the passivation layer 220 is formed on the sidesurface 213 and a top surface 215 of the first patterned conductivelayer 212 a. The annealing process AP is performed at about 60° C.˜about300° C. for about 10˜about 60 minutes, for instance, and gas applied inthe annealing process AP is oxygen, nitrogen, a mixture of oxygen andnitrogen, or any other appropriate gas. According to the presentembodiment, the annealing process AP is performed at about 250° C. forabout 30 minutes, for instance.

With reference to FIG. 2D, a light emitting layer 230 is formed on thefirst electrode 210 a. According to the present embodiment, the lightemitting layer 230 is formed on the passivation layer 220, for instance.Here, the light emitting layer 230 may include a red organic lightemitting pattern, a green organic light emitting pattern, a blue organiclight emitting pattern, a light emitting pattern with other colors, or acombination of the aforesaid light emitting patterns. Besides, the lightemitting layer 230 is formed by vacuum evaporation, for instance. Toimprove the light emitting efficiency of the light emitting device, ahole transporting layer 222 is further formed between the firstelectrode 210 a and the light emitting layer 230 according to thepresent embodiment, and the hole transporting layer 222 is formed byvacuum evaporation, for instance. Whether the hole transporting layer222 is formed may be determined based on actual design demands andshould not construed as a limitation to the invention.

With reference to FIG. 2E, a second electrode 240 is formed on the lightemitting layer 230. The material of the second electrode 240 may bereferred to as that described in the previous embodiment. Besides,according to an embodiment of the invention, an electron transportinglayer, an electron injection layer, or both layers may be furtherdisposed between the second electrode 240 and the light emitting layer230.

The light emitting device 200 in the present embodiment includes thesubstrate 202, the first electrode 210 a, the passivation layer 220, thelight emitting layer 230, and the second electrode 240. The firstelectrode 210 a is disposed on the substrate 202 and includes a firstpatterned conductive layer 212 a. The first patterned conductive layer212 a includes an alloy containing the first metal and the second metal.According to this embodiment, the first electrode 210 a further includesa second patterned conductive layer 214 a located between the substrate202 and the first patterned conductive layer 212 a. The passivationlayer 220 is at least disposed on the side surface 213 of the firstelectrode 210 a and includes the compound of the second metal. Here, thework function of the compound of the second metal ranges from about 4.8to about 5.5, for instance. In the present embodiment, the passivationlayer 220 is further disposed on the top surface 215 of the firstelectrode 210 a. The second electrode 240 is disposed on the firstelectrode 210 a. The light emitting layer 230 is disposed between thefirst electrode 210 a and the second electrode 240. According to thepresent embodiment, the light emitting device 200 further includes thehole transporting layer 222, for instance.

In the present embodiment, the passivation layer 220 is formed on theside surface 213 and the top surface 215 of the first electrode 210 athrough performing the annealing process AP on the first electrode 210a. Since the passivation layer 220 located on the side surface 213 ofthe first electrode 210 a prevents the first electrode 210 a from beingexposed to a stripper or other substance and thus protects the firstelectrode 210 a from being corroded, the first electrode 210 a can havefavorable device properties and long service life. In addition, thepassivation layer 220 located on the top surface 215 of the firstelectrode 210 a provides an interface with the high work function, so asto improve the hole transporting efficiency at the interface between thefirst electrode 210 a and the hole transporting layer 222. Thereby, thelight emitting device 200 can be characterized by favorable deviceproperties and light emitting efficiency.

It should be mentioned that three evaporation processes and threepatterning processes need to be performed on the material layers inorder to form an electrode with a three-layer structure (e.g.,ITO(bottom)/silver(middle)/ITO(upper)) according to the related art.Nonetheless, according to the present embodiment, the passivation layeris formed on both the side surface and the top surface of the electrodethrough performing the annealing process. This not only achievesequivalent effects to those accomplished by the uppermost conductivelayer in the conventional electrode but also prevents exposure of thesidewall of the electrode. Namely, at least one evaporation process andone patterning process may be omitted in the present embodiment incomparison with the related art. Hence, the manufacturing method of thelight emitting device in the embodiment has simplified manufacturingsteps, and the resultant light emitting device is characterized byfavorable device properties, satisfactory light emitting efficiency, andlong service life.

FIG. 3A to FIG. 3D are schematic cross-sectional flow chartsillustrating a manufacturing method of a light emitting device accordingto an embodiment of the invention. The manufacturing method of the lightemitting device described in the present embodiment is similar to thatof the light emitting device depicted in FIG. 2E, while the differencetherebetween mainly lies in the method of forming the first electrodeand the passivation layer, which will be elaborated hereinafter. Thematerials and manufacturing methods of the same components are alreadydescribed in the previous embodiment and thus will not be reiteratedbelow. With reference to FIG. 3A, a first electrode material layer 210is formed on a substrate 202. According to this embodiment, the firstelectrode material layer 210 includes a second conductive layer 214, afirst conductive layer 212, and a third conductive layer 216sequentially disposed on the substrate 202. Note that it is likely forthe first electrode material layer 210 not to include the secondconductive layer 214 in another embodiment of the invention. The firstconductive layer 212 includes an alloy containing a first metal and asecond metal. According to the present embodiment, the first conductivelayer 212, for instance, includes silver-indium alloy, aluminum-indiumalloy, silver-zinc alloy, aluminum-zinc alloy, silver-indium-zinc alloy,aluminum-indium-zinc alloy, or any other alloy. A material of the secondconductive layer 214 is ITO, zinc oxide, or zinc-tin oxide, forinstance. A material of the third conductive layer 216 is ITO, zincoxide, or zinc-tin oxide, for instance. The first, second, and thirdconductive layers 212, 214, and 216 are respectively formed byperforming an evaporation process or a sputtering process.

With reference to FIG. 3B, the first electrode material layer 210 ispatterned to form a first electrode 210 a. According to this embodiment,a method of patterning the first electrode material layer 210 includessequentially patterning the third conductive layer 216, the firstconductive layer 212, and the second conductive layer 214, for instance.The first electrode 210 a includes a second patterned conductive layer214 a, a first patterned conductive layer 212 a, and a third patternedconductive layer 216 a sequentially disposed on the substrate 202.According to the present embodiment, the first electrode 210 a has thethree-layer structure containing ITO/silver-indium alloy/ITO, forinstance.

With reference to FIG. 3C, an annealing process AP is performed on thefirst electrode 210 a, so as to form a passivation layer 220 on a sidesurface 213 of the first electrode 210 a. Here, the passivation layer220 includes a compound of the second metal. A work function of thepassivation layer 220 ranges from about 4.8 to about 5.5 in the presentembodiment, for instance. The passivation layer 220 includes InO_(x),zinc oxide, or indium-zinc oxide, for instance. It should be mentionedthat the top surface of the first patterned conductive layer 212 a is,for instance, covered by the third patterned conductive layer 216 a inthe present embodiment, and therefore the passivation layer 220 isformed on the exposed side surface 213 of the first patterned conductivelayer 212 a. Nevertheless, according to an embodiment (not shown), giventhat the third patterned conductive layer 216 a includes reactive metal,the passivation layer 220 may be further formed on the exposed topsurface of the third patterned conductive layer 216 a.

With reference to FIG. 3D, a light emitting layer 230 is formed on thefirst electrode 210 a, and a second electrode 240 is formed on the lightemitting layer 230. According to the present embodiment, a holetransporting layer 222 is further formed between the first electrode 210a and the light emitting layer 230. The materials of the light emittinglayer 230, the second electrode 240, and the hole transporting layer 222and the method of forming the same may be referred to as those describedin the previous embodiment and thus will not be reiterated herein. Notethat the hole transporting layer 222 may be omitted in anotherembodiment.

In the present embodiment, the first electrode 210 a in the lightemitting device 200 a has a three-layer structure containing theconductive layer 214 a/the conductive layer 212 a containing alloy/theconductive layer 216 a, and the passivation layer 220 is formed on theside surface 213 of the electrode 210 a. In addition, the conductivelayer 216 a disposed in the upper portion of the first electrode 210 aprovides an interface with the high work function, so as to improve thehole transporting efficiency at the interface between the firstelectrode 210 a and the light emitting layer 230 or the holetransporting layer 222. Since the passivation layer 220 prevents thefirst electrode 210 a from being exposed to a stripper or othersubstance and thus protects the first electrode 210 a from beingcorroded, the first electrode 210 a can have favorable device propertiesand long service life. Thereby, the light emitting device 200 can becharacterized by favorable device properties and light emittingefficiency.

In light of the foregoing, the electrode in the light emitting devicedescribed in an embodiment of the invention includes the alloycontaining the first metal and indium, zinc, or indium-zinc, and indium,zinc, or indium-zinc accounts for about 0.1 wt %˜about 2 wt % of thealloy. Thereby, the device properties and the light emitting intensityof the light emitting device can be significantly improved.

In the light emitting device and the manufacturing method thereof, asdescribed in another embodiment of the invention, the annealing processis performed on the electrode that includes the alloy containing thefirst metal and the second metal, such that the passivation layercontaining the compound of the second metal is formed on the sidesurface of the electrode. As such, the electrode is not exposed anddamaged. The passivation layer may be further formed on the top surfaceof the electrode, so as to provide an interface with a proper high workfunction between the light emitting layer and the electrode and furtherimprove the hole transporting efficiency at the interface between theelectrode and another layer. As such, the device properties and thelight emitting intensity of the light emitting device can besignificantly improved. From another perspective, the manufacturingmethod of the light emitting device described herein can be easilyintegrated into the existing manufacturing process in no need ofpurchasing additional equipment. Accordingly, the manufacturing costs ofthe light emitting device herein do not significantly increase.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the structure of theinvention without departing from the scope or spirit of the invention.In view of the foregoing, it is intended that the invention covermodifications and variations of this invention provided they fall withinthe scope of the following claims and their equivalents.

What is claimed is:
 1. A manufacturing method of a light emittingdevice, comprising: forming a first electrode on a substrate, the firstelectrode comprising a first patterned conductive layer, the firstpatterned conductive layer comprising an alloy containing a first metaland indium, an alloy containing the first metal and zinc, or an alloycontaining the first metal and indium-zinc, wherein indium, zinc, orindium-zinc accounts for about 0.1 wt %˜about 2 wt % of the alloy;performing an annealing process on the first electrode to form apassivation layer at least on a side surface of the first electrode,wherein the passivation layer comprises a compound of indium oxide, zincoxide, or indium-zinc oxide; forming a light emitting layer on the firstelectrode; and forming a second electrode on the light emitting layer.2. The manufacturing method of the light emitting device of claim 1,wherein the first metal comprises silver or aluminum.
 3. Themanufacturing method of the light emitting device of claim 1, whereinthe first patterned conductive layer comprises silver-indium alloy,aluminum-indium alloy, silver-zinc alloy, aluminum-zinc alloy, or acombination thereof.
 4. The manufacturing method of the light emittingdevice of claim 1, wherein the passivation layer is further formed on atop surface of the first electrode.
 5. The manufacturing method of thelight emitting device of claim 1, wherein the first electrode furthercomprises a second patterned conductive layer disposed between the firstpatterned conductive layer and the substrate.
 6. The manufacturingmethod of the light emitting device of claim 5, wherein the secondpatterned conductive layer comprises indium-tin oxide, zinc-tin oxide,or indium-zinc oxide.
 7. The manufacturing method of the light emittingdevice of claim 1, wherein the first electrode further comprises asecond patterned conductive layer and a third patterned conductivelayer, and the first patterned conductive layer is disposed between thesecond patterned conductive layer and the third patterned conductivelayer.
 8. The manufacturing method of the light emitting device of claim7, wherein the second patterned conductive layer and the third patternedconductive layer comprise indium-tin oxide , zinc-tin oxide, orindium-zinc oxide.
 9. The manufacturing method of the light emittingdevice of claim 1, further comprising forming a hole injection layerbetween the first electrode and the light emitting layer.